The broad objective is to determine the neural and hormonal factors which are responsible for or which influence the normal contractile and milk secretion functions of the mammary gland in relationship to the incipient intense and declining stages of lactation. I propose to: (1) continue to investigate the mechanisms controlling the active regulation of prolactin reaccumulation in the pituitary following its discharge by suckling and continue to investigate the suckling and exteroceptive mechanisms for prolactin release. Prolactin will be analyzed by standard electrophoretic-densitometric and radioimmunoassay techniques. (2) continue to investigate the factors which influence the stimulatory effect of prolactin upon refilling of the mammary gland with milk. (3) continue to investigate the physical, neural and hormonal factors which influence the contraction of the mammary gland myoepithelium in response to oxytocin. Intramammary pressure recordings and in vivo estimates of milk yield are the end points used in these studies.